Farmers Adiya, left, and Baiyunde, right, stand in front of a state-of-the-art power plant that turns millions of tons of coal every year into methane in northern China’s Inner Mongolia province.

HEXIGTEN, China — Deep in the hilly grasslands of remote Inner Mongolia, twin smoke stacks rise more than 200 feet into the sky, their steam and sulfur billowing over herds of sheep and cattle.

Both day and night, the rumble of this power plant echoes across the ancient steppe, and its acrid stench travels dozens of miles away.

This is the first of more than 60 coal-to-gas plants China wants to build, mostly in remote parts of the country where ethnic minorities have farmed and herded for centuries. Fired up in December, the multibillion-dollar plant bombards millions of tons of coal with water and heat to produce methane, which is piped to Beijing to generate electricity.

It’s part of a controversial energy revolution China hopes will help it churn out desperately needed natural gas and electricity while cleaning up the toxic skies above the country’s eastern cities. However, the plants will also release vast amounts of heat-trapping carbon dioxide, even as the world struggles to curb greenhouse gas emissions and stave off global warming.

If all of the plants start up, the carbon dioxide they’d release would equal three-quarters of all energy-related carbon emissions in the U.S., according to U.S. government data and energy experts from Duke and Stanford universities. That is far more than now produced in China by burning coal, the country’s main source of power.

So far, China is running only two pilot plants to produce methane, which is also known as synthetic natural gas, in the provinces of Inner Mongolia and far western Xinjiang, with another 21 approved. Building all 60 plants would cost an estimated $65 billion.

“Once you have invested in it, China will have locked itself in a high water-consuming, high carbon-emitting path,” said Chi-Jen Yang, the Duke energy researcher. “This short-term mistake will become a mistake that will be hard to turn around for decades.”

Chinese leaders are under intense pressure to meet rising energy needs spurred by economic growth but are hampered by insufficient reserves of natural gas and oil.

At the same time, China’s massive cities are contending with air pollution so intense it can shut down schools and airports and, studies show, shorten life expectancy by as many as five years.

Central to the appeal of the coal-to-gas plan is that it moves polluting energy production far away from cities while also turning the country’s vast coal reserves into more valuable natural gas.

Yet scientists at Tsinghua University and Ford Motor Co. estimate the process emits between 36 and 82 percent more greenhouse gases than burning coal to produce electricity. The resulting methane can also be used to power vehicles, heat homes and cook food.

Already, China emits more heat-trapping carbon into the atmosphere than any other nation and twice that of the U.S., the world’s second biggest carbon emitter. Even without the new plants, China’s carbon emissions are projected to double over the next 25 years, while U.S. emissions stay steady, U.S. Energy Information Administration data show.

“Everybody wants to find a path forward to solve the problems of (synthetic natural gas) and at the same time solve China’s pollution problems,” said Yanling Gong, editor-in-chief of the government-affiliated China National Chemical Information Centre.

While less directly polluting than burning coal, the Inner Mongolia plant run by state-owned company Datang has already transformed this corner of rural Hexigten county famous for its long-maned horses and sun-burnt vistas.

As a boy growing up there, farmer Adiya could ride his horse through waist-high grass for miles without meeting another person. Now, the 32-year-old says he stays indoors some mornings because of the industrial stench.